Composite pellets of carbon and prereduced iron oxides have been recognized as a highly beneficiated charge for an electric smelting furnace or a blast furnace for the production of metallic iron. Such composite pellets comprised of iron in the metallic state, iron oxides, gangue constituents, and carbon can be produced by a number of processes, of which some have been described as both metallized and carbonized pellet products and exemplified by such patents as U.S. Pat. Nos. 2,805,141 to Apuli, 2,806,779 to Case, 3,218,153 to Schei et al, 3,219,436 to Heitmann et al, 3,333,951 to Ban, 3,264,091 to Ban, 3,264,092 to Ban, 3,276,859 to Collin et al, 3,304,168 to Ban, 3,751,241 to Sloughfy et al, and 3,759,693 to Kunii et al.
The present invention is an improvement on all of the foregoing although the most important of the references known at this time is the U.S. Pat. to Case No. 2,806,779. This patent contemplates the preparation of hardened iron ore pellets by forming within the pellet a "char bond" of graphite material. Ranges of concentration of the char bonding carbonaceous material utilized in specific examples recited by Case overlap the ranges of concentration of nonagglomerating carbonaceous material utilized in accordance with the present invention. Although mention is made of categories of carbonaceous materials which include nonagglomerating coals or lignites or sub-bituminous materials, tests embodying the teachings of Case have shown that unless the material within these categories is of the agglomerating type, it will not form a char bond and the resulting product is not comparable to that which is produced in accordance with the present invention.
Case, it will be noted, in describing his invention described an amount of carbon to be used in the following language:
"By `conglomerated body` herein is meant a coherent mass containing an intimate mixture of ore and reducing agent so proportioned to one another that the self-sufficient condition is attained: Such terms being employed herein to define the mass in its heat-treated form referred to as a `char-bonded pellet` or `char-bonded materials`."
The term "self-sufficient" refers to "self-reducing" as defining a chemical ratio in the char-bonded pellets by which enough carbon is intimately mixed with the iron ore to reduce to metallic form the iron oxide contained in the char-bonded pellets under the conditions of reduction in the smelting furnace.
The present invention is distinguished from the Case patent in these important respects: First, in the present invention a nonagglomerating carbonaceous material is utilized in forming the pellets. As indicated above, tests have shown that only agglomerating materials have the capability of forming a char bond. Secondly, the amount of carbon utilized in the present process is from 40 - 80% of the amount of carbon required (1) to carburize the iron produced in the present process and (2) to reduce all of the iron oxide constituents to the metallic state. Although these sub-self-sufficient amounts are overlapped by amounts taught by Case in specific examples, those specific examples do not achieve the desired result according to Case of providing a self-reducing pellet and there is no indication as to whether the coal used in the tests was of the agglomerating (bituminous) or nonagglomerating type. Third, the prior art produces a char bond of "graphitic material". In the present case, there is no tendency to produce a char bond and the amount of carbon in the pellet is limited so as not to interfere with the formation of a metallic bond and/or a ceramic bond within the pellet. There is no suggestion in Case that by limiting the amount of coal in the pellet, a bond other than char bond can be produced under sintering conditions, to produce a strong "durable" pellet. Moreover, there is no suggestion that the carbon deficiency can be supplied as external coal as hereinafter explained to produce a composite smelter charge composition.
With respect to U.S. Pat. No. 2,805,141 to Apuli, the present invention is clearly distinguished therefrom with respect to the relative amounts of carbon employed. Apuli is concerned with the production of iron oxide pellets, not a smelter charge. Thus there is an improper carbon:oxygen ratio. As will appear hereinafter, a carbon content of about 3.6% by weight of the pellet will give a carbon to oxygen ratio of about 0.11. In the present invention the carbon to oxygen ratio is approximately 0.8 or higher. Secondly, in the production of iron ore pellets, the pellets are produced under oxidizing conditions. In the present invention, reducing conditions are maintained. It will be observed that Apuli calls for oxidizing gases in the burning zone.
The patent to Kunii utilizes the combination of a traveling grate in which to produce pellets and a rotary kiln which in reality replaces a blast furnace. Coal is introduced separately into the rotary kiln. Coal is mixed with the hot pellets from the traveling grate to prereduce the iron oxide in the hot pellets. Coke which is produced in accordance with the process may be recycled. The patentee's process is distinguished from the traveling grate process and the rotary hearth process in that the former has the disadvantage that the material of the grate makes it very difficult to maintain the grate at a high reducing temperature over a prolonged period of time and thus renders the process unsuitable for providing highly metallized pellets. The latter poses a problem in that it cannot be used for effecting reduction of pellets.
The patent to Collin is concerned with a reduction process utilizing an electric arc furnace. He first produces pellets which contain substantially no metallic iron. In the present invention, at least about 30% iron is produced in the pellets. Quite importantly, however, Collin adds coke to the pellets at the time of introduction into the furnace. In the present invention, coke is produced simultaneously with the hardened prereduced pellets, and the material which is discharged from the traveling grate is a metallurgically complete blended smelter furnace charge.
The patentee Heitmann et al is concerned with the production of low density pellets which, after prereduction, are crushed and briquetted to provide a portion of a smelter furnace charge.
In the Sloughfy et al patent, slag is used in excess to coat the pellets and prevent reoxidation of prereduced iron on weathering. This composition is then fired in a rotary kiln with added coke. As indicated, the present invention contemplates the production of a metallurgically self-sufficient charge from a traveling grate machine suitable for direct introduction into a smelting furnace without further compositional adjustment. The patentee Schei contemplates increasing the yield of metal by molding the pellets or briquettes in a manner to provide a core which contains an excess of metal oxide surrounded by a second layer or pile which contains an excess of carbonaceous reducing agent. Both the core and the shell contain iron ore. All of the fuel is therefore within the pellet and none is external.
It is believed that in those cases where a full measure of carbon is utilized in the formation of a pellet and the carbon is of the nonagglomerating type such as may be used in accordance with the Ban patents above referred to, the presence of the relatively large amount of carbon interferes with the establishment of an internal matrix of metal and/or an internal matrix of ceramic material which will hold the pellet together.
The importance of holding the pellet together when using a low grade type carbonaceous material such as the lignites or sub-bituminous coals or extremely high grade coal such as anthracite coal (all of which materials are nonagglomerating) is that when these materials are used in accordance with the Case teachings, there results a very loosely bonded material which will crumble under pressure of the fingers. However, when pellets formed from the same materials and heated under the same conditions are made using a deficiency of a nonagglomerating carbonaceous material, the resulting pellets are extremely strong and can be handled without crumbling into fines. Thus, materials moving on grates or being dumped into a smelting furnace shaft maintain their integrity and do not crumble into fines which fall through grates on the traveling grate machine or contaminate and render inefficient the smelting furnace.
The invention is further distinguished from the prior art in that it provides a charge for a smelting furnace as distinct from merely pellets. Because a deficiency of carbon is used in forming the pellet moiety, the charge to the smelting furnace is nevertheless made self-sufficient by the addition of loose non-agglomerating carbonaceous material in the interstices between pellets or as a coating around the pellets. Also, the charge may desirably contain a fluxing material such as limestone or dolomite. The fluxing material may be present either in the pellet composition or in the interstitial composition or in both. Nevertheless, the resulting product for charging to the smelting furnace is formed from cheap raw materials and after sintering has excellent handling properties.
As indicated in much of the prior art referred to above, a problem has been encountered in the treating of pellets on a traveling grate with reoxidation. In accordance with the present invention, a reducing environment is maintained in the bed during the metallizing procedure. The formation of a pyrolytic char or coke as a reductant for electrothermal processing simultaneously is a distinct advantage and difference over the prior art. When the burden discharged from the traveling grate is cascaded into a supply hopper for introduction into an electrothermal smelting furnace, the char continues to provide a protective reducing environment so that the pellets in the smelter charge composition do not undergo reoxidation. A still further advantage of the present invention is that it enables the operator to provide corrective carbon much more rapidly than heretofore possible. When a deficiency of carbon is detected in the final product of the electrolytic smelting furnace, additional "corrective coal" can be introduced immediately into the process. The entire processing in accordance with the present invention enables the production of a hot metallurgical self-sufficient coke/hardened prereduced pellet composition available for immediate introduction into an electric arc smelting furnace.